The civil war rebellion was in full motion. You can blame the previous presidents that punted on the issue for years. It depends on what your perspective of "terrible things" is. we also killed lots of nazis to take hitler out of power. Had Lincoln not assumed control of the union and done everything he could to squash the rebellion, it would have set the stage for a divided north and south, continuing conflict between the two sides, and the allowance for the south to continue the disgusting practice of slavery.

What other option was there but war?

The industrial revolution would have ended slavery just as certainly as the war did, only without half a million casualties.